I rebased my change on top of master to retrieve PR #10626 bugfix.

@ncoghlan, @serhiy-storchaka, @pitrou, @zooba, @ericsnowcurrently: Would you mind to review this PR? I created a new Include/unstable/ subdirectory for all "#ifndef Py_LIMITED_API" APIs. See https://bugs.python.org/issue35134 for the rationale.

I plan to merge this PR next week.

@ncoghlan: You approved the PR but proposed a different directory name. Are you ok with "unstable" name? https://bugs.python.org/issue35134#msg330261

Note: this PR is my second attempt, the first one was PR #10285 which wasn't properly designed.

There's no real easy way to test it, but Tools/msi/dev/dev.wixproj is going to need its <InstallFiles Include="$(PySourcePath)include\*.h"> line changed to <InstallFiles Include="$(PySourcePath)include\**\*.h"> (or a semi-colon separated list of directories to include in the installer).

Better to make the change now and cause the installer build to break than to accidentally ship without necessary headers. If it doesn't work, I'll fix it up.

@zooba: Aha, I was looking for the code of the Windows installer.

Tools/msi/dev/dev.wixproj is going to need its line changed to

Right now, Include/internal/ is not installed (on Unix at least). I just proposed to install these headers as well: https://bugs.python.org/issue35296

Would it be possible to only update the MSI installer once this PR and PR #10665 are merged?

I renamed the subdirectory from Include/unstable/ to Include/cpython/.

@vstinner If you're willing to keep reminding me to do it once this is merged until it's done, sure.

@vstinner If you're willing to keep reminding me to do it once this is merged until it's done, sure.

I merged my PR #10665 (make install now installs the internal API) and I udpated this PR to apply your proposed change in MSI (copy Include/**/*.h).

@vstinner While I preferred "cpython" as the directory name, I was OK with "unstable", hence the original approval. I'm happy you decided to switch the name anyway, though :)